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ComputerAided_Design_Engineering_amp_Manufactur.pdf

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FIGURE 1.13 Tolerance synthesis schema.<br />

Assembly graph: The assembly graph describes the procedure for assembling the different parts in<br />

the assembly without considering the effect of tolerances. At present, the assembly graph is specified<br />

by the user.<br />

• Phase #2 (Generation): In this phase, the PFM model for the part surfaces, along with the process<br />

model for the part, is generated. The PFM model represents the equivalent part functions on the<br />

different surfaces of the part. The PFM model for a surface is essentially a collection of kinematic<br />

and force degrees of freedom (dof), along with other part behavior attributes. The kinematic and<br />

force dofs represent the absence/presence of motions and transmission of forces along a particular<br />

axis of the surface. 16 The process model represents the process plan for manufacturing the part<br />

without considering the effect of tolerances. 15<br />

• Phase #3 (Synthesis): The different tolerances for the part are synthesized at this stage. The<br />

synthesis procedure consists of two subphases: (a) transformation of the PFM model into functional<br />

tolerance limits, and (b) constraining the functional tolerance limits with respect to different<br />

constraints. The first and second subphases take extensive support of the component functionfunctional<br />

tolerance limit and geometric entity-process error relationship knowledge bases, and<br />

the process-tolerance limit data base. The size and form tolerances are assigned on the surfaces<br />

while the positional tolerances are assigned on the location of different geometric features in the<br />

part. The datum reference planes/features for the different geometric features of the part are also<br />

assigned at this phase. The data are assigned on the basis of the PFM assignments for the different<br />

surfaces of the part in conjunction with their manufacturability, assemblability, and inspectability<br />

criteria.<br />

• Phase #4 (Assignment): After the assignment of data at the part level, different form tolerances<br />

have to be redefined with respect to their datum reference planes. In this stage, the form tolerances<br />

are substituted with different orientation tolerances using a form-to-orientation tolerance conversion<br />

chart. 16 After completion of this stage, the different tolerances are assigned to the product<br />

model of the part.<br />

© 2001 by CRC Press LLC

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